An adequate conception of mind and its relationship to matter should explain how it is possible for mental events to interact with the rest of the world, and in particular to themselves have a causal influence on the physical world. It is easy to think that this must be impossible: it takes a physical cause to have a physical effect. Yet everyday experience and theory alike show that it is commonplace. Consciousness could hardly have evolved if it had had no uses. In general, it is a measure of the success of any theory of mind and body that it should enable us to avoid epiphenomenalism.